The existing public-switched telephone network (PSTN) comprises: a large number of central office switches for serving local subscriber lines and trunks; a smaller number of tandem (also referred to as "toll" or "transit" switches) for providing connections between central office switches; a plurality of analog and digital communication paths for interconnecting the central office switches and tandem switches; optional databases for storing information; and a common channel signaling (CCS) system for transmitting messages used by the switches to establish call connections. The CCS system includes many inter-connected signal transfer points (STPs) which relay messages among network nodes to establish, manage, and release call paths in the PSTN. To facilitate accurate message delivery, each PSTN switch is assigned a discrete "point code" for identification purposes.
Calls are routed by an "originating" central office switch (that is, the central office switch that serves the caller) to a "destination" central office switch (the central office switch that serves the called party) on the basis of the directory number received in the originating switch from the caller. In the United States, directory numbers are assigned in accordance with a numbering plan requiring each subscriber line to be identified by ten digits. The ten digit directory number comprises a three digit area code which defines a specific geographic region followed by a three digit office code which identifies a particular central office switch in the network. The office code is followed by a four digit subscriber line identifier which establishes the network address of a particular subscriber line on the central office switch associated with the office code digits.
In accordance with current protocol, a call is routed over a call path by the originating switch subsequent to CCS messages being relayed between the originating switch, other network nodes, and the destination switch. Particularly, an initial address message (IAM) is extended from the originating switch to the destination switch, either directly or via tandem switches, to establish trunk connections using a process well-known in the art. The call path between the originating switch and the destination switch is completed by a series of switch "hand-offs" of the call over the established trunk connections. In other words, the originating switch performs both routing (selecting a call path) and switching (establishing call connections on the selected call path) functions to connect a call to a desired called party upon receipt of a dialed directory number.
It is the dual role of determining routes and performing switching functions, however, that induces the inefficient operation of present-day telecommunications networks. This is because the determination of the call path is made by the originating switch and subsequently, any intermediate switches. A problem arises if unforeseen system blockages necessitate an alternate call path. In such cases, the call must be extended back to the originating switch from the node at which the blockage is detected so that the originating switch can devise a new call path. Return of the blocked call to its origination point causes undue delay and taxes network resources. Therefore, there is a need in the art to effectively and efficiently make routing determinations in a telecommunications network.